The Arches Cluster Mass Function

We have analyzed H and Ks–band images of the Arches cluster obtained using the NIRC2 instrument on Keck with the laser guide star adaptive optics (LGS AO) system. With the help of the LGS AO system, we were able to obtain the deepest ever photometry for this cluster and its neighborhood, and derive the background-subtracted present-day mass function (PDMF) down to 1.3M⊙ for the 5–9 annulus of the cluster. We find that the previously reported turnover at 6M⊙ is simply due to a local bump in the mass function (MF), and that the MF continues to increase down to our 50 % completeness limit (1.3M⊙) with a power-law exponent of Γ = −0.91 for the mass range of 1.3 < M/M⊙ < 50. Our numerical calculations for the evolution of the Arches cluster show that the Γ values for our annulus increase by 0.1–0.2 during the lifetime of the cluster, and thus suggest that the Arches cluster initially had Γ of −1.0 ∼ −1.1, which is only slightly shallower than the Salpeter value. Subject headings: Galaxy: center — open clusters and stellar associations: general — stars: luminosity function, mass function — celestial mechanics, stellar dynamics